Baluns circuits are used to make a balanced to unbalanced transformation, for impedance matching between the balanced and unbalanced ports, and also suppress the common mode signal at the balanced ports at the 2nd harmonic frequency.
One example of a widely known conventional solution (utilized by Chipcon AS of Norway as well) is illustrated in FIG. 2. In the circuit of FIG. 2, LC resonant circuits at the two signal paths of the differential outputs/inputs (O1b 58 and O2b 60) shift +90 and −90 degrees at the fundamental frequency and thus sum the two signal paths in phase at the single ended output 72. In this conventional approach, the matching network is typically difficult to design if a parallel parasitic impedance is present between the differential outputs/inputs, e.g. a transmitter (TX) or receiver (RX) with parallel parasitic RC.
At resonance, which is the normal operating condition, the network should present an optimum antenna impedance at the differential inputs (O1b 58 and O2b 60) in order to deliver maximum power to the unbalanced port 72 in Tx mode or in a reverse way in Rx mode. This optimum antenna impedance has a reactance (usually inductive) that resonates with the TX or RX reactance (usually capacitive) at the operating frequency. The impedance also has a real part that must be properly adjusted to obtain a voltage swing between the differential inputs up to the allowed maximum or if the voltage swing is not maximized to produce a conjugate matched termination for the TX or RX. One problem with this matching network is that it operates well only near to the series resonant frequency of it's LC (L1 62, C1 64) and CL (C2 66, L2 70) arms. Thus, the network shows a high impedance between its differential inputs (O1b 58 and O2b 60) so that the generation of the optimum antenna impedance is difficult if not impossible.
Further, this approach typically requires a highly filtered supply voltage rail Vcc to act as a high quality RF ground, which is illustrated in FIG. 2 as inductor L1 62 connected to Vcc. In addition, there is no common mode suppression in the solution of FIG. 2. If a common mode 2nd harmonic trap is introduced, then the trap will influence the fundamental resonant frequency, which complicates the design process.
Another example of a conventional solution is found in U.S. Pat. No. 6,529,075 for “Amplifier with Suppression of Harmonics” by Yuri Bruck, Gennady Burdo, and Michael Zelikson, which issued Mar. 4, 2003. In this example, an amplifier with differential input and output is shown. Odd and even sample circuits, respectively, are used to sample the odd and even harmonic currents. Using these sample currents, a compensation circuit generates odd and even compensation signals, which are subtracted from the odd and even output harmonic signals in order to reduce their amplitude level. Both the circuit input and output in this example are differential. So the common mode suppression circuit core does not realize a balun function with a single ended output.
Another example of a conventional solution is found in U.S. Pat. No. 3,821,655 for a “High Frequency Amplifier” by Alan J. Fisher, issued Jun. 28, 1974. In this patent, a 2nd harmonic trap is used at the differential amplifier output. The trap consists of two series LC resonators, one on each signal path. On each signal path, the LC resonators are formed by the series collector parasitic inductance of the applied bipolar transistor and by a parallel capacitor to the ground. The LC resonators resonate at the 2nd harmonic frequency and thus form a short circuit from each signal path to the ground potential. The filtered differential signal is transformed to a single ended signal by an ideal coil balun.
In Fisher, the circuit uses the series parasitic inductances of the transistors with external capacitors. Also, the 2nd harmonic suppression and the balun operation (coil balun) are realized with different circuit blocks, which adds cost to the circuit. The solution of Fisher may be applied in the case of transistors with series parasitic output inductance, but is generally not applicable to differential transmitters with parallel parasitic capacitances. Also, the differential transmitter outputs are not matched to the coil balun inputs at the fundamental frequency, which causes losses that reduce power transfer.
Another example of a balun circuit is found in U.S. Pat. No. 7,034,630 for a “Balun transformer and transceiver”. In this reference, a balun circuit is described that achieves the balanced-unbalanced transformation by using coupled transmission line elements. It also includes two resonant circuits: one at the single ended and one at the differential side. They generate a band pass filter characteristic at the fundamental frequency.
Still another example of a conventional solution is found in U.S. Pat. No. 6,658,265. In this patent, a dual mode amplifier with a matching and power combining network is described. It is capable of operating in common mode at one frequency band and differential mode at another frequency band. The matching network is configured to maintain the same input and output impedances regardless the type of operation (common mode or differential mode). The matching network is also configured to terminate the second harmonic for each band without affecting the fundamental tone at the other band. This circuit realizes the 2nd harmonic suppression and the power combining function, which is the balun function in case of the differential signal, to a single ended antenna with separate circuit blocks. The circuit of this reference appears to have been designed to address complicated applications and the solution itself is, therefore, more complex than is necessary or desirable in simpler or lower cost applications.